Alexander Szabo and Oscar Engle - Svenskt Vatten

COD concentration, theoretical=er channels contributes with 60-90% of the total incoming
water to the WSP. It can be seen on the velocity measurements that soon after the rainfall occur,
the flow speed increases rapidly. As the rainfall ends, the flow tends to go down quickly. On the
18-19 of November, most students left UTM for holiday. Approximately one half to two thirds of
all students were not at the University during these measurements. Despite this, both Q max and
Q min (Table 8.4) were higher during the holiday than during the school period. This could be
explained by that the monsoon period started just after the measurements on the 5-6 November,
but before the 18 th of November. More or less every day received heavy rainfalls which could
have raised the groundwater level, thus creating more infiltration into the sewer system. Since the
storm water channels are built to receive all the storm water, no additional water should enter the
sewage network. If the infiltrated water could be reduced, the HRT in the WSP would increase,
hence improving the treatment efficiency. An unavoidable effect of reducing the infiltration is the
increase in concentrations of pollutants in the waste water. It is however preferable to receive
smaller quantities of waste water, even if the concentrations of pollutants are higher, since with
effective reduction the total load of pollutants on the recipient will be lower than with diluted
waste water.
Sampling procedure for influent water
The influent water was collected every two hours during the measurement periods. Samples were
withdrawn from the inlet chamber and immediately taken to the laboratory and stored in a
refrigerator.
The first sampling started 5 th November at 17:30 and continued every second hour until 14:15 the
next day. The second sampling started at 12:30 a.m. on the 18 th of November and continued every
second hour until 10:00 a.m. the next day. The third sampling started at 12:00 a.m. on the 9 th of
January and continued every second hour to 10:00 a.m. the 10 th of January.
From all these samplings days COD, BOD 5 and TSS were analyzed according to the methods
described in chapter 7. From the samples 5-6 November k-values were analyzed. Since the
measurements during 5-6 November lacks flow data between 17:30 and 22:00, a designed flow
was created and used in order to get complete measuring data and to calculate weighted
parameters (see Figure 8.10).
BOD 5 and COD in influent water
The inlet COD concentrations show a tendency to rise when people use water consuming
facilities at most, usually in the morning and late afternoon. When water flow increases due to
rainfall, the COD values peaks and reaches levels much higher than during normal flow. At late
night or early morning (04:00-06:00) all three sampling periods reach their lowest values (12 mg/l
– 33 mg/l) (Figure 8.11). The pattern of COD concentrations is lower during the holiday period,
which is explained by the lower load due to absence of students in combination with infiltration.
According to Mara (2003), a COD concentration of less than 400 mg/l is considered as “weak
strength”. This water is therefore placed into this category.
From the first sampling period 5-6 November, BOD 5 from all 12 measurements were analyzed.
For all 12 measurements that period, COD was also analyzed. This was done in order to get a
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